The present invention relates to a method for continuous cooking of lignocellulosic fibre material, comprising the steps of sequentially in a first stage, impregnating the fibre material in an impregnation liquid comprising alkali metal hydroxide, and thereafter withdrawing a spent impregnation liquid; in a second stage, cooking the fibre material in a cooking liquor comprising alkali metal hydroxide; in a third stage, adding, to said fibre material, a liquid which is rich in hemicellulose, said liquid preferably comprising at least a part of said withdrawn spent impregnation liquid.
The object of the method according to the invention is to reintroduce hemicellulose, especially xylan, which is present (dissolved) in the spent impregnation liquid, in contact with the fibre material, so that hemicellulose is precipitated onto the fibres for improved yield and beatability of the pulp which is produced by the method.
Besides cellulose, lignocellulosic fibre raw material comprises lignin and hemicellulose. The essential purpose of the conventional chemical digestion process of fibre material is to remove the lignin from the fibre material to produce a pulp of cellulose. It is however beneficial to, to a large extent, retain the hemicellulose in the pulp, since the presence of hemicellulose improves yield and beatability. Especially the presence of xylan improves beatability. Xylan is the main hemicellulose in hardwood, i.e. about 30% of the dry wood material, while softwood contain only about 10% xylan. Contrary to cellulose, xylan does not show a crystalline structure, but is branched, which means that it is in principle water soluble. Xylan is especially dissolved into a hydroxide ion containing solution, due to its carboxylic acid groups. These carboxylic acid groups will however be successively cleaved off during the digestion, which leads to the solubility being decreased, especially if also the concentration of hydroxide ions in the solution is decreased during the digestion process.
From U.S. Pat. No. 3,617,431 there is known a process for preparing cellulose pulp by alkaline digestion while inhibiting extraction of hemicellulose by employing an alkaline cooking liquor having a high concentration of substantially nondegraded hemicellulose. In the process, said alkaline cooking liquor is withdrawn from a location in the upper part of the digester, whereafter it is cooled and introduced in the top of the digester.
U.S.Pat. No. 3,802,956 is presenting a method for impregnation of cellulosic fibre material, where the fibre material is preimpregnated in a preimpregnation vessel which includes two screen sections with a counter current impregnation zone there between and extraction of spent impregnation liquid at the upper screen section. The method includes the possibility to introduce a part of this spent impregnation liquid at a location which corresponds to the extraction screen in a subsequent continuous digester in order to utilise a possible alkali content thereof. The method is not concerned with the problem of retaining hemicellulose in the pulp and will moreover not have this effect. This is understood by the stage which precedes the digester extraction screen being a concurrent stage and the stage which succeeds the digester extraction screen being a counter current washing stage, which means that the spent impregnation liquid which is introduced at the location of the digester extraction screen must leave the digester through this screen and consequently will not have any retention time worth mentioning in the digester.
The present invention is concerned with the problem of achieving good yield and beatability by keeping a high content of hemicellulose, especially xylan in the pulp. It would be especially beneficial to have the xylan on the outside of the fibres, since the xylan molecules on different fibres thereby would be able to co-operate with each other to give good beatability. It also desired to achieve a cost effective method to produce a pulp with a low kappa number while preserving good strength properties, while, if possible, excluding the counter current wash stage which conventionally is combined with so called modified continuous cooking, extended modified continuous cooking or isothermal cooking (ITC(trademark)).
The above problem is solved by allowing hemicellulose which is dissolved during impregnation of the fibre material to precipitate on the fibres in a subsequent stage of the digestion process. This is achieved by a method for continuous cooking of lignocellulosic fibre material, comprising the steps of sequentially
(a) in a first stage, impregnating the fibre material in an impregnation liquid comprising alkali metal hydroxide, and thereafter withdrawing (14, 514) a spent impregnation liquid,
(b) in a second stage, cooking the fibre material in a cooking liquor comprising alkali metal hydroxide,
(c) in a third stage, adding (A), to said fibre material, a liquid which is rich in hemicellulose, said liquid preferably comprising at least a part (14a, 514a) of said withdrawn spent impregnation liquid, characterised by
(d) in a fourth stage, cooking the fibre material in a cooking liquor comprising the liquid added in the third stage, whereby the fibre material is subjected to a retention time of at least 1 hour in said fourth stage.
According to one aspect of the invention, said impregnation liquid in said first stage (a) essentially consists of spent cooking liquor.
According to another aspect of the invention, said fourth stage (d) is performed with a retention time of at least 1,5 hours.
According to another aspect of the invention, a fresh liquid comprising alkali metal hydroxide, preferably a white liquor, is added to said second stage (b), so that said cooking liquor in stage (b) obtains a concentration of effective alkali, calculated as NaOH, of at least 20 g/l, preferably 30-50 g/l and more preferred about 40 g/l, for softwood, or at least 10 g/l, preferably 12-25 g/l and more preferred about 15 g/l, for hardwood. Also, a fresh liquid comprising alkali metal hydroxide, preferably a white liquor, can be added to the fibre material together with said hemicellulose rich liquid in stage (c).
According to yet another aspect of the invention, a first spent cooking liquor is withdrawn from said second stage (b) and a substantial part of this first spent cooking liquor, preferably at least 80%, more preferred at least 90% and optimally about 100%, is supplied to the impregnation in said first stage (a), preferably to the beginning of said first stage (a). Possibly, the spent impregnation liquid which is added to the first stage (a) may provide all the alkali metal hydroxide necessary for that stage, there being no need for any addition, or possibly only need for a minor addition, of fresh alkali metal hydroxide to the first stage (a).
According to another aspect of the invention, said hemicellulose rich liquid, which constitutes a part of the cooking liquor in the fourth stage (d) of the method, may be transferred from an impregnation stage in a fibre line for digestion of hardwood fibre material, to a continuous fibre line system for digestion of softwood fibre material. Although the embodiments which are described in connection with the drawings show only one fibre line, it is to be understood that the preferred features regarding for example retention times, temperatures, flows etc. would be applicable also in the case of two fibre lines.
According to still another aspect of the invention, the method can be performed in connection with continuous chemical digestion, preferably a kraft digestion process, which employs a so called single vessel system or a so called two vessel system, which systems may be of hydraulically liquid filled type or of steam/liquor-phase type. Moreover, the preimpregnation vessel in a two vessel system operates preferably in a concurrent way, i.e. both the fibre material and the liquid flow in the same direction, possibly with somewhat different speed however. The digester, on the other hand, may independent of type, operate with certain zones being concurrent and other being counter current, whereby it is preferred that the entire digester, apart from the very last wash stage, which has a retention time of at most about 60 minutes, but preferably at least about 10 minutes, operates in a concurrent manner. This means that the design which have been used for the last decades, including a counter current zone, often called modified continuous cooking, extended modified continuous cooking or ITC(trademark) , below the extraction screens, in some cases can be abandoned.